Method and system for high-speed, precise micromachining an array of devices

ABSTRACT

A method and system for high-speed, precise micromachining an array of devices are disclosed wherein improved process throughput and accuracy, such as resistor trimming accuracy, are provided. The number of resistance measurements are limited by using non-measurement cuts, using non-sequential collinear cutting, using spot fan-out parallel cutting, and using a retrograde scanning technique for faster collinear cuts. Non-sequential cutting is also used to manage thermal effects and calibrated cuts are used for improved accuracy. Test voltage is controlled to avoid resistor damage.

CROSS-REFERENCE TO RELATED PATENTS AND APPLICATIONS

This is a divisional of application(s) Ser. No. 10/397,541 filed on Mar.26, 2003, now U.S. Pat. No. 6,951,995, entitled Method and System ForHigh-speed, Precise Micromachining an Array of Devices which claimspriority of Ser. No. 60/368,421 filed Mar. 28, 2002 and which is acontinuation-in-part application of U.S. patent application Ser. No.10/108,101 entitled “Method For Processing a Device, Method and SystemFor Molding Same and the Device” filed Mar. 27, 2002, now U.S. Pat. No.6,972,268.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and systems for high-speed, precisemicromachining of an array of devices. This invention also relates tothe field of resistor trimming, particularly trimming of serpentineresistors on thin film block structures. Laser resistor trimminginvolves laser-processing cuts in an area of resistive material betweenconductors.

2. Background Art

Resistor trimming (as well as trimming and micro-machining of otherelectronic components and circuits) has evolved over the laser 20 yearsand is now used for adjusting circuits for thick film, thin film, andother electronic technologies. The publication “Trimming,” LIA HANDBOOKOF LASER MATERIALS PROCESSING, Chapter 17, pages 583-588, 2001 containscontributions describing several aspects of laser trimming. FIGS. 1 a-1c of the present application are incorporated from the publication. FIG.1 a illustrates current flow lines of an untrimmed resistor, whereasFIG. 1 b illustrates an effect of laser trimming on the current flowlines. FIG. 1 c summarizes several results (stability, speed, andtolerances ) with various resistor geometries and cut types.

The following exemplary U.S. patents are related to laser trimmingmethods and systems: U.S. Pat. Nos. 6,510,605; 6,322,711; 5,796,392;4,901,052; 4,853,671; 4,647,899; 4,511,607; and 4,429,298.

U.S. Pat. No. 4,429,298 relates to many aspects of serpentine trimming.Basically, a serpentine resistor is formed with sequential plunge cutsand a final trim cut is made parallel to the resistor edge from the lastplunge. It describes “progressively” making plunge cuts on a resistoralternately from one end, considers maximum and minimum plunge cutlengths, a resistance threshold of the plunge cuts for the trim cut, afaster cutting speed for plunge cuts, and a structured process flow withvarious resistance and cut length tests.

There is a continuing need for improved high-speed, micromachining suchas precise trimming at all scales of operation, ranging from thick filmcircuits to wafer trimming.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved method andsystem for high-speed, precise micromachining an array of devices.

In carrying out the above object and other objects of the presentinvention, a method is provided for high-speed, precise micromachiningan array of devices. Each of the devices has at least one measurableproperty. The method includes the steps of: a) selectivelymicromachining a device in the array to vary a value of a measurableproperty; b) suspending the step of selectively micromachining; c) whilethe step of selectively micromachining is suspended, selectivelymicromachining at least one other device in the array to vary a value ofa measurable property; and d) resuming the suspended step of selectivelymicromachining to vary a measurable property of the device until itsvalue is within a desired range.

The devices may be resistors.

The resistors may be film resistors.

The steps of selectively micromachining may be performed with at leastone laser beam which cuts the devices.

The method may further include measuring one measurable property of atleast one of the devices to obtain a measured value.

The method may further include comparing the measured value with apredetermined threshold to obtain a comparison and micromachining atleast one of the other devices based on the comparison.

The method may further include selectively micromachining at least oneof the other devices based on the measured value.

The method may further include determining not to measure a measurableproperty of at least one of the other devices based on the measuredvalue.

The method may be for high-speed, precise laser trimming an array ofresistors, and one of the measurable properties may be resistance.

Each of the steps of selectively micromachining may include the step ofselectively removing material.

The array may include at least one of one or more rows and one or morecolumns.

At least one of the steps of selectively micromachining may be performedwith a plurality of focused laser pulses to irradiate a plurality ofdevices substantially simultaneously.

At least one of the steps of selectively micromachining may be performedwith a plurality of focused laser pulses. The method may further includedistributing the focused laser pulses.

The step of distributing may include the steps of producing adistribution pattern with a plurality of laser beams and focusing thelaser beams.

The laser trimming may produce a series of interdigited cuts in an areaof resistive material between conductors of the resistors.

At least one of the steps of selectively micromachining may include thesteps of positioning a laser beam at a location of each of the devicesto be micromachined and selectively irradiating at least a portion ofeach of the devices to be micromachined with at least one laser pulse.

At least one of the steps of selectively micromachining may include thesteps of generating and relatively positioning a laser beam to travel ina first direction within a field of the array and selectivelyirradiating at least a portion of at least one device within the fieldwith at least one laser pulse.

The method may further include generating and relatively positioning alaser beam to travel in a second direction substantially opposite thefirst direction within the field and selectively irradiating at least asecond portion of at least one device within the field with at least onelaser pulse.

At least one of the steps of selectively micromachining may include thesteps of generating and relatively positioning a laser beam to travel ina first scanning pattern across the devices, superimposing a secondscanning pattern with the first scanning pattern and irradiating atleast one device with at least one laser pulse.

The second scanning pattern may be a retrograde scan, and scan speed ofthe at least one laser pulse irradiating the at least one device islower than a corresponding scan speed of the first scanning pattern.Laser energy may be concentrated at the at least one device for a periodof time longer than a period of time associated with only the firstscanning pattern whereby throughput is improved.

The second scanning pattern may include a jump from a first device to asecond device.

The steps of selectively micromachining may be performed with aplurality of laser pulses, and at least one of the pulses may have anenergy in the range of 0.1 microjoules to 25 millijoules.

The measured value may be a measured temperature value.

The devices may be substantially identical.

Further in carrying out the above objects and other objects of thepresent invention, a system is provided for high-speed, laser-based,precise micromachining an array of devices. Each of the devices has atleast one measurable property. The system includes a pulsed lasersubsystem. An optical subsystem is coupled to the pulsed laser system toselectively irradiate a portion of a device with a laser pulse. Acontroller is coupled to the subsystems to control the subsystems to: a)selectively micromachine a device in the array to vary a value of ameasurable property; b) suspend the selective micromachining; c) whilethe selective micromachining is suspended, selectively micromachine atleast one other device in the array to vary a value of a measurableproperty; and d) resume the selective micromachining to vary ameasurable property of the device until its value is within a desiredrange.

The optical subsystem may include a beam deflector and a beam deflectorcontroller for controlling the beam deflector to scan a laser beam alonga first scan pattern which includes each of the devices to bemicromachined.

The system may further include a measurement subsystem to measure one ofthe measurable properties of at least one of the devices.

The micromachining may be laser trimming and the array is an array ofresistors, and the measurement subsystem may be a probe array.

The optical subsystem may include a second beam deflector to superimposea higher-speed, second scan pattern on the first scan pattern wherebythroughput of the system is improved.

The controller may be coupled to the subsystems so that the subsystemsare controlled to generate a trimming sequence for at least one of thedevices that reduces device temperature during micromachining.

The above object and other objects, features, and advantages of thepresent invention are readily apparent from the following detaileddescription of the best mode for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a-1 b are schematic views which illustrate current flow linesbefore and after laser trimming, respectively;

FIG. 1 c is a chart which illustrates the effect of various cut types onseveral trim parameters;

FIG. 2 a is a schematic view of an array of chip resistors arranged inrows and columns and which illustrates results using laser trimmingsteps in accordance with an embodiment of the present invention;

FIG. 2 b is a block diagram flow chart further defining trimming stepscorresponding to FIG. 2 a;

FIG. 3 is a block diagram flow chart further defining the trimmingoperations of FIGS. 2 a and 2 b in a system of the present invention;

FIG. 4 a is a schematic view of an array of chip resistors arranged inrows and columns and which illustrates results using laser trimmingsteps in accordance with another embodiment of the present invention;

FIG. 4 b is a block diagram flow chart further defining trimming stepscorresponding to FIG. 4 a;

FIG. 5 is a block diagram flow chart further defining the trimmingoperations of FIGS. 4 a and 4 b in a system of the present invention;

FIG. 6 a is a schematic view of a laser trimming system which may beused in at least one embodiment of the invention;

FIG. 6 b is a schematic view of a resistor which has geometricproperties to be measured, specifically edges of the resistor, usingdata obtained with the system of FIG. 6 a;

FIG. 7 is a graph which shows position of a laser beam versus timeduring scanning of a resistor array in one embodiment wherein a fastscan with a solid state deflector is superimposed with aelectromechanical linear scan to selectively form the cuts of eitherFIG. 2 or FIG. 4 at increased speed;

FIG. 8 is a schematic view of a system delivering multiple focused beamsto at least one resistor so as to increase trimming speed; and

FIG. 9 is a schematic view of a system which provides multiple beams toat least one resistor in a laser trimming system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

High-Speed Serpentine Trimming Process

In resistor trimming, the cuts direct the current flowing through theresistive material along a resistance path. Fine control and adjustmentof the cut size and shape change the resistance to a desired value, asillustrated in FIG. 1 a-1 c. Typically, chip resistors are arranged inrows and columns on a substrate. FIG. 2 a shows an arrangement wherein arow of resistors R1,R2, . . . RN is to be processed. A probe array,having a probe 200 and depicted by arrows in FIG. 2 a, is brought intocontact 202 with the conductors of a row of resistors. A matrix switchaddresses the contacts for a first pair of conductors (e.g.: contactsacross R1) and a series of cuts and measurements is performed to changethe resistance between the conductor pair to a desired value. When thetrimming of a resistor is complete, the matrix switches to a second setof contacts at the next row element (e.g.: R2) and the trimming processis repeated. When a complete row of resistors (R1 . . . RN) has beentrimmed, contact is broken between the contacts and the probe array. Thesubstrate is then relatively positioned to another row, the probe arrayis brought into contact, and a second row is processed in the manner asthe preceding row.

The trimming of serpentine thin film resistors, for instance asillustrated in FIG. 1c, involves laser processing to createinterdigitated cuts in an area of resistive material between conductors.The interdigitated cuts direct current flowing through the resistivematerial along a serpentine path that wraps around the cuts. Thisgeometry allows a wide range of resistances to be created with a singleareal film/conductor layout. The approach outlined above would process asequence of serpentine cuts with measurement steps at a resistor siteand then move to the next resistor.

Referring to FIG. 2 a, an initial laser position for any cut is depictedas 205, and a beam positioner directs the beam along the linear paththrough the resistor material. In accordance with the present invention,a new paradigm trims a leg on a first resistor (e.g. trim cut 204 of R1)and measures the resistance. If the resistance is below a predeterminedthreshold, similar collinear trims across other resistors R2 . . . RN inthe row are made. A completed collinear trim along the row isillustrated at 210 in FIG. 2 a, and the corresponding block 220 isfurther defined in FIG. 2 b. In at least one embodiment of the presentinvention, a subset of resistors may be measured to determine thin filmconsistency across the substrate, but if the thin film is of knownconsistency one measurement may be sufficient.

The next collinear group of cuts along resistors of the row is made inthe same manner as shown at 211 of FIG. 2 a and further defined at block221 of FIG. 2 b, the resistor RN being trimmed initially. The process isrepeated as shown in 212-213 of FIG. 2 a with corresponding furtherdefined at blocks 222-223 of FIG. 2 b. If a measurement shows that athreshold has been crossed, trimming of the row R1 . . . RN proceedswith measurement of each resistor so as to trim to value beforeswitching to the next resistor (depicted as 214 at block 224).

Limiting the number of measurements and maintaining a collinear trimtrajectory both increase trim speed.

The flowchart of FIG. 3 further defines steps, corresponding to FIGS. 2a-2 b, and additional processing steps used in a trimming system (e.g.:indexing and loading).

In at least one embodiment, cutting steps may be carried out based uponpre-determined information. By way of example, for some resistor types,a first series of elements may be cut before resistance is measured, thesequence based on pre-determined parameters of the resistor (e.g.:geometry) and/or known film properties, (e.g: sheet resistance).Similarly, a number of non-measured cuts may be determined in a learnmode at the first resistor (e.g: including at least one measurement, oriterative measurements). In one learn mode, iterative measurements aremade and the number of non-trim cuts is determined based on themeasurements and material properties. In at least one embodiment anumber of the non-measured cuts may be calculated.

For example, four cuts may be made without measurement. Referring toFIG. 4 a, an initial condition 410 is illustrated wherein probes areplaced in contact 202 with the row as in FIG. 2 a. Referring to FIG. 4b, the initial condition is further defined at block 420. By way ofexample, FIG. 4 a-4 b illustrate an embodiment of the trimming processwherein initially four cuts 411 are made without any measurement. Asshown in FIG. 4 b, block 421 defines a predetermined number of cuts(e.g: four), without measurement, based on at least one pre-trim valueor condition. The scan path for completing four cuts is depicted at 405.Then the first resistor R1 in the row is trimmed at 406 and measured todetermine if the target value is reached. If not, the remainingresistors R2 . . . RN are cut (e.g.: without measurement) as depicted at412, further defined by block 422.

Then the process is repeated, beginning with trimming 407 of RN, andthen cutting of R[N−1] to R1 as shown at 413 and further defined byblock 423. Hence, with each change in direction either R1 or RN istrimmed, and if the target value is not reached the remaining resistorsR2 . . . RN or R[N−1] . . . R1, respectively, are cut. A final stepresults after R1 or RN reaches a target value. Each resistor isconnected and trimmed sequentially, illustrated at 414 and furtherdefined by block 424.

The flowchart of FIG. 5 further defines steps corresponding to FIG. 4a-4 b, and additional processing steps used in a trimming system (e.g.:which includes the steps of indexing and loading).

In one embodiment, wherein pre-determined information is obtained usingiterative measurements, pre-trim values are provided. The values may bespecified by an operator, process engineer, or otherwise obtained. Thesoftware provides capability for specifying or using the pre-trim targetvalues so that the applied test voltage and/or current is controlled.This feature is useful for avoiding voltages that are high enough todamage the part over the wide range of resistance change associated withserpentine trimming. When using a fast resistor measurement system in anembodiment of the invention, the voltage applied to the resistor formeasurement is decreased for the initial low resistance cut to limitcurrent through and potential damage to the resistor. As subsequent cutsare made and the resistance increases, the measurement voltage isincreased.

The exemplary trim and cut sequences of FIGS. 2 a and 2 b and 4 a and 4b may be modified so as to allow for variations in material propertiesand other process parameters and tolerances.

For example, in at least one embodiment of the present invention,additional steps may be utilized when a measured trim cut reaches thetarget value and length is within a predetermined margin of the maximumallowed cut length. Within the margin, variation in the materialproperties may leave some trim cuts short of the target value andrequire additional cuts.

In a first mode, trim cuts are made sequentially in a row of elementsand the location of elements not reaching the target value are saved.With subsequent trim cuts, the remaining elements at the saved locationsare trimmed to the target value.

In a second mode, based on the length of the first element trimmed tovalue, the cut length is reduced to prevent the target value from beingreached and non-measurement cuts are processed to complete the row.Subsequent trim cuts bring all elements in the row to the target value.

In a third mode, the length of at least one prior cut on an element ismodified to prevent subsequent cuts from falling into the marginalcondition.

In at least one embodiment, additional steps may be utilized when thevalue of a measured trim cut is within a predetermined margin of thetarget value. Within the margin, variation in the material propertiesmay leave some elements beyond the target value using fullnon-measurement cuts.

In a first mode, trim cuts are made sequentially in a row of elementsand the location of elements not reaching the target value are saved.With subsequent trim cuts, the remaining elements at the saved locationsare trimmed to the target value.

In a second mode, based on the value measured in the first element, thecut length is reduced to prevent the target value from being reached andnon-measurement cuts are processed to complete the row. Subsequent trimcuts bring all elements in the row to the target value.

In a third mode, the length of at least one prior cut on an element ismodified to prevent subsequent cuts from falling into the marginalcondition.

Experimental data indicates improvements in throughput by cutting allthe resistors in a row as shown in FIG. 2-4, as opposed to theconventional single resistor trim technique. By way of example,approximate results are shown in the table below:

32 RESISTOR ROW, 20 CUTS PER RESISTOR Laser Q-Rate (KHz) Single ResistorTrim (Sec) Row Trim (Sec) 5 39 28 10 27 16 20 20 10

The overall trim speed increases with an increasing number of resistorsin a row, fewer measurements, and with reduced time for final (i.e.,fine) trimming.

Further, each resistor has additional time to recover from lasergenerated energy. The sequence of cuts may be determined to managetemperature change in an element (e.g. reduce maximum elementtemperature during cutting). For example, with reference to FIG. 4 a,the sequence 405 may be reversed so that a set of cuts are made startingnear the center of an element and progressing to an end of the elementapproaching the conductor and probe. Other sequences, suitable sequencesmay be used (e.g: any sequence of non-adjacent cuts having advantage forthermal management). Preferably, a second element may be cut prior to anadditional step of measuring.

Range of resistance change for serpentine cuts varies from about 1 orderof magnitude (e.g: 10×), two orders of magnitude typical (100×), and upto about 500× with current materials.

Laser Trimming Systems

In at least one embodiment of the invention a laser trimming system maybe first calibrated using a method as described in “Calibrating LaserTrimming Apparatus”, U.S. Pat. No. 4,918,284. The '284 patent teachescalibrating a laser trimming apparatus by controlling a laser beampositioning mechanism to move a laser beam to a desired nominal laserposition on a substrate region, imprinting a mark (e.g., cutting a line)on a medium to establish an actual laser position, scanning theimprinted mark to detect an actual laser position, and comparing theactual laser position with the desired nominal position. Preferably, thelaser beam operates on one wavelength, and the mark is scanned with adetection device that operates on a different wavelength. The detectiondevice views a field that covers a portion of the overall substrateregion, and determines the position of a mark within the field. The '284patent further teaches determining where a beam position is in relationto a camera field of view.

Other calibration techniques may be used alone or in combination withthe '284 method. For instance, U.S. Pat. No. 6,501,061 “LaserCalibration Apparatus and Method,” discloses a method of determiningscanner coordinates to accurately position a focused laser beam. Thefocused laser beam is scanned over a region of interest (e.g. anaperture) on a work-surface by a laser scanner. The position of thefocused laser beam is detected by a photodetector either atpredetermined intervals of time or space or as the focused laser beamappears through an aperture in the work surface. The detected positionof the focused laser beam is used to generate scanner position versusbeam position data based on the position of the laser scanner at thetime the focused laser beam is detected. The scanner position versusbeam position data can be used to determine the center of the apertureor the scanner position coordinates that correspond with a desiredposition of the focused laser beam.

Subsequent to system calibration, which preferably includes calibrationof numerous other system components, at least one substrate havingdevices to be trimmed is loaded into the trimming station.

Referring to FIG. 6 a, partially incorporated from the '284 patent, animproved laser trimming system may include an infrared laser 602,typically having a wavelength from about 1.047 microns-1.32 micronswhich outputs a laser beam 603 along an optical path 604 to and througha laser beam positioning mechanism 605 to a substrate region 606. Forapplication to trimming of thin film arrays, a preferred wavelength ofabout 0.532 microns may be obtained by doubling the output frequency ofthe IR laser using various techniques known in the art and commerciallyavailable.

The laser beam positioning mechanism 605, preferably includes a pair ofmirrors and attached respective galvanometers 607 and 608 (variousavailable from the assignee of the present invention). The beampositioning mechanism 605 directs the laser beam 603 through a lens 609(which may be telecentric or non-telecentric, and preferablyachromatized at two wavelengths) to a substrate region 606, over afield. The X-Y galvanometer mirror system may provide angular coverageof the entire substrate if sufficient precision is maintained.Otherwise, various positioning mechanisms may be used to providerelative motion between the substrate and the laser beam. For instance,a two-axis precision step and repeat translator illustratedschematically as 617 may be used to position the substrate within thefield of galvanometer based mirror system 607,608 (e.g.: in the X-Yplane). The laser beam positioning mechanism 605 moves the laser beam603 along two perpendicular axes thereby providing two dimensionalpositioning of the laser beam 603, across the substrate region 606. Eachmirror and associated galvanometer 607, 608 moves the beam along itsrespective x or y axis under control of a computer 610. Illuminationdevices 611 which may be halogen lights or light emitting diodes producevisible light to illuminate substrate region 606.

A beam splitter 612 (a partially reflective mirror) is located withinthe optical path 604 to direct light energy reflected back along thepath 604 from the substrate region 606 to a detection device 614. Thedetection device 614 includes a camera 615, which may be a digital CCDcamera (e.g.: color or black/white) and associated frame grabber 616 (ordigital frame buffer provided with the camera), which digitizes thevideo input from the television camera 615 to obtain pixel datarepresenting a two-dimensional image of a portion of the substrateregion 606. The pixel data are stored in a memory of the frame grabber616, or transmitted, for instance, by a high speed link, directly to thecomputer 610 for processing.

The beam positioning subsystem may include other optical components,such as a computer-controlled, optical subsystem for adjusting the laserspot size and/or automatic focusing of the laser spot at a location ofthe substrate.

In applying the invention to thin film trimming of resistor arrays, atleast one thin film array is supported by the substrate. The calibrationdata obtained as above is preferably used in combination with anautomated machine vision algorithm to locate an element (e.g. resistorR1) of the array and measure the location of at least one geometricfeature of an element 620 of FIG. 6 b. For instance, the feature may beone of the horizontal edges 621 (e.g.: an edge parallel to theX-direction), and one of the vertical edges 622 (e.g.: an edge parallelto the Y direction) found by analysis of pixel data in memory using oneof numerous available edge detection algorithms. The edges may includemultiple edge measurements along the entire perimeter of a resistor, asample of the edges, or edges from numerous resistors of the array. Thewidth of the resistor is then determined which may be used to define thecutting length, typically as a predetermined percentage of the width.Preferably, the edge information is obtained automatically and used withcalibration data to control the length of each cut within the row R1 . .. RN, for example. Other measurement algorithms may also be used wheresuitable, for instance image correlation algorithms or blob detectionmethods.

Calibration may be applied at one or more points along the cut. In atleast one embodiment the starting point of at least one cut will becorrected with calibration data.

Preferably, the length and the starting point of a plurality of cuts inFIGS. 2 and 4 will be corrected.

Most preferably, the length and starting point of all cuts in FIGS. 2 aand 4 a will be corrected.

In one embodiment, the first resistor (e.g.: R1 or RN) will becalibrated, and a corresponding correction applied to all resistors(e.g.: R1, . . . , RN) of the row.

Complete automation is preferred. However, a semi-automatic algorithmwith operator intervention may be used, for instance where agalvanometer is positioned so that the array element 620 is in thefield, then the beam is sequentially positioned along the elementinteractively and the intensity profile (or derivative or intensity)observed on a display 630 by an operator.

The use of the calibration information to adjust coordinates within thearray region is valuable for improving the precision of laser beampositioning without throughput degradation. Measurements of resistorwidth and the alignment data is useful for both controlling the lengthof a cut and for correcting deviations from linearity andnon-orthogonality of the array relative to the scanner X,Y coordinatesystem. The use of the calibration data for geometric correction isparticularly well suited for use in laser trimming systems having one ormore linear translation stages.

Geometric correction does not necessarily replace other useful systemdesign features including f-theta lens linearity, fan beam compensationetc. The system tolerance stack-up may generally be used to determinetradeoffs between the number of cut calibration locations based onexpected position error. When fanning out the beam, especially withlarge spacing across many resistors, only one is calibrated and aligned.For instance, when the spacing between resistors is relatively large, asingle cut may be calibrated and aligned. Resulting errors in positionare anticipated at elements, to be mitigated in part with system design,f-theta linearity, fan spread compensation etc. Closely spaced cuts of atransverse fan are expected to have smaller errors compared with on axisfan.

Further Throughput Improvements—Optical Techniques

In at least one embodiment of the present invention the throughput maybe further improved by increasing the effective scan rate using one ormore of the techniques below.

Further increases in processing speed with collinear trims can beaccomplished with faster jumps across trim gaps between the resistors ofa row. One such gap 216 is shown in FIG. 2 a. Referring to FIG. 7, in atleast one embodiment of the invention, a single-axis Acousto-Optic BeamDeflector (AOBD) superimposes a saw tooth linear scan pattern 701 as thegalvanometer scans across the row at a constant velocity 702. Duringtrimming the AOBD scans in retrograde motion 703, and, between trims,provides a fast jump 704 to the next cut. This allows the galvanometerto scan at constant velocity and minimizes the contributions of jumps tothe total process time.

The use of acousto-optic deflectors in combination with galvanometersfor speed improvements is known in the art. For instance, U.S. Pat. No.5,837,962 discloses an improved apparatus for heating, melting,vaporizing, or cutting a workpiece. A two-dimensional acousto-opticdeflector provided about a factor of five improvement in marking speed.

U.S. Pat. No. 6,341,029, which is incorporated by reference in itsentirety, shows in FIG. 5 thereof an embodiment having severalcomponents which may be used in a complete system when practicing thepresent invention in a retrograde mode for increased speed. In the '029patent, acousto-optic deflectors and galvanometers, with an associatedcontroller, are shown for dithering CW beams for laser patterning. Alsosee col. 3, line 47 and col. 4 of the '029 patent for additional detailsregarding system construction.

The arrangement of the '029 patent may be readily adapted, usingavailable techniques, so as to provide modifications of opticalcomponents and scan control profiles so as to practice the retrogradescanning technique of the present invention, preferably with additionalhardware calibration procedures.

In another embodiment of the invention, the collinear trims onserpentine resistors may be accomplished in a parallel fashion withmultiple spots along the row. A fan-out grating or other multi-beamgenerating device is used to create a spot array so that 2 or more spotsare formed and aligned according to the resistor pitch along the row.For example, U.S. Pat. No. 5,521,628 discloses the use of diffractiveoptics to simultaneously mark multiple parts. The multiple beams may belower power beams generated from a more powerful laser source, orcombined beams from multiple sources. The scan system scans the multiplebeams and forms spots through a common scan lens simultaneously acrossmultiple resistors. The trim process is similar to the single spotmethod with two or more cuts in parallel during non-measurement cuttingsteps. When the threshold is reached, the system converts to a singlespot mode to serially trim each resistor to value.

Similarly, the collinear trims on serpentine resistors may beaccomplished in a parallel fashion with multiple spots formed on atarget to make parallel cuts. A fan-out grating or other multi-beamgenerating device is used to create a spot array so that 2 or more spotsare formed, the spots being aligned to an element with predeterminedspacing between cuts. If a predetermined number of cuts are performed(e.g. four as shown in FIG. 4 a) then, in one embodiment, the number ofpasses could be reduced by 50% (e.g.: a single pass in each direction).This embodiment may be most useful if resistor process variations andtolerances are well established. The grating may be in an opticallyswitched path so as to selectively form multiple spots or a single spot.

Published U.S. patent application No. 2002/0162973 describes a methodand system for generating multiple spots for processing semiconductorlinks for memory repair. Various modifications in the lens system anddeflector system may be used to generate multiple spots for use in thepresent invention.

In one embodiment, a single laser pulse is used to trim up to tworesistors at one time (e.g., no, one or two cuts). Referring to FIG. 8,two focused spots 801,802 are formed on two cuts by spatially splittingthe single collimated laser beam 803 into two diverging collimated beams804,805. Fine adjustment of the differential frequency controls spotseparation. The use of acousto-optic devices for spatially splittingbeams in material processing applications is known in the art. Forexample, Japanese patent abstract JP 53152662 shows one arrangement fordrilling microscopic holes using a multi-frequency deflector havingselectable frequencies f1 . . . fN.

A laser 806 of FIG. 8 is pulsed at a predetermined repetition rate. Thelaser beam goes through relay optics 807 that forms an intermediateimage of the laser beam waist into the acoustic optic modulator (AOM)aperture. The AOM 808, which operates in the Bragg regime, preferably isused to controllably generate the two slightly diverging collimatedfirst order diffraction laser beams and control the energy in each beam.The AOM is driven by two frequencies, f1 and f2 where f1=f0+df andf2=f0−df where df is a small percentage of the original RF signalfrequency f0. The angle between the two beams is approximately equal tothe Bragg angle for f0 multiplied by 2 (df/f0). The AOM controls theenergy in each of the laser beams by modulating the signal amplitudes oftwo frequency components, f1 and f2, in the RF signal 812 and makingadjustments for beam cross-coupling.

After exiting the AOM 808, the beams go through an optional beamrotation control module 809 to rotate the beam 90 degrees so as toorient the beam in either X or Y. In one embodiment, a prism is used forthis rotation, though many rotation techniques are well known asdescribed in related U.S. patent publication No. 2002/0170898.

Next, the beam goes through a set of optics to position the beam waistand set the beam size to be appropriate for the zoom optics andobjective lens 810. The zoom optics also modify the angle between thetwo beams, therefore the angle between the two beams exiting the AOM 808has to be adjusted depending on the zoom setting to result in thedesired spot separation at the focal plane. Next, the laser beams enterthe objective lens 810 which provides a pair of focused spots 801,802 ontwo resistors. The two spots are separated by a distance that isapproximately equal to the focal length of the lens 810 times the anglebetween the two beams. The retrograde and parallel methods can becombined for collinear trimming on serpentine resistors. For example, abeam is scanned by an AOBD then split into a pair and scanned across thefield. Two adjacent resistors are trimmed simultaneously and the jump isfrom resistor N to resistor N+2 to the next pair or resistors.

Alternatively, or with a two-dimensional deflector, a pair of spots maybe produced in a direction orthogonal to the serpentine scan direction.For instance, with relatively simple control and programming of aone-dimensional AOBD, the deflector may be used (with appropriate outputpower control) to simultaneously produce at least two of the four beamsused for making four cuts as shown in FIG. 4 a. As such, the scan timefor the cuts may be reduced by 50%. As a result of programmabledeflection, the AOBD may be preferred over a fan out grating. Themultiple spots may also be produced during coarse and fine trim asneeded.

FIG. 9 illustrates schematically an exemplary embodiment of an improvedlaser trimming system having a module 901 from FIG. 8 added for eitherretrograde scanning, parallel processing, or a combination thereof. Forexample, a signal 902 from the computer 610 may be used to control theAOBD or other solid state deflector 808 in one or more axes, and thebeam rotation module 809, if provided. The module 901 may include relayoptics 807 and other beam shaping components. Preferably, at least oneAOBD is used so as to provide considerable flexibility and ease of use,for example with a digital RF generator providing the control signal 812from the computer 610.

Furthermore, techniques for forming elongated or elliptical spots can beemployed with this invention to further increase processing speed orquality. Improvements in trimming speed associated with spot shaping aredescribed in co-pending published U.S. patent application No.2002/0170898.

Numerous other design alternatives may be used in at least oneembodiment of the invention for enhancing system performance and ease ofuse. For example, alternatives include but are not limited to thefollowing:

1. The system may provide for computer-controlled spot size and/or focusadjustments. U.S. Pat. No. 6,483,071, assigned to the assignee of thepresent invention, illustrates an optical subsystem providing for bothspot size control and dynamic focus for laser based memory repair.

2. Another alternative is control of beam energy with a variable beamattenuator. The attenuator may be an acousto-optic deflector (ormodulator). Neutral density filters or polarization-based attenuatorsmay be used, whether manually or automatically adjusted. In U.S. Pat.No. 6,518,540 a suitable variable attenuator is shown, by way ofexample, having a rotating half waveplate and a polarization-sensitivebeam splitter.

3. The pulse width may be varied using methods known to those skilled inthe art, with the understanding that the energy of a q-switched laserwill vary with repetition rates, particularly at high repetition rates.For dynamic trimming, wherein a measurement is performed between pulses,it may be preferred to maintain substantially constant pulse energy. Amethod for pulse energy control is disclosed in the U.S. Pat. No.6,339,604 which reduces the variation in energy at the target as thetrimming speed is decreased (e.g.: larger pulse temporal spacing),corresponding to periods of precision measurement when the resistancevalue approaches the pre-determined target value.

4. In at least one embodiment, a diode-pumped, frequency-doubled, YAGlaser is used to trim the resistor array. The output wavelength of 532nm resulted in low drift, absence of microcracking, and negligible heataffected zone when compared to other wavelengths. A pulse width of about25-45 ns may be preferred, with less than 30 ns typical. The preferredmaximum laser repetition rate will be at least 10 KHz. The pulse width,much less than typical for thick film systems, provides for thin filmmaterial removal at a relatively high repetition rate. Preferably, themaximum available pulse energy at the reduced pulse widths and highrepetition rates will allow for losses associated with the diffractiveoptics (e.g: grating or AOBD) so that multiple spots may be provided.

5. The laser may be focused to an approximate, diffraction-limited, spotsize. The spot size will typically be less than about 30 microns orless, with a preferred spot size less than about 20 microns, and a mostpreferred spot size in the range of about 6-15 microns, for instance,10-15 microns.

6. In the illustrated embodiments of the invention, serpentine cuts areillustrated as a series of parallel interdigitated cuts. However, it isto be understood that application of the present invention is notrestricted to forming parallel cuts. Trimming or micromachining so as toproduce a plurality of non-intersecting cuts with a reduced number ofmeasurements is considered to be within the scope of the invention.

7. Further, embodiments of the invention are not restricted to thin filmresistor measurements, but are applicable to other micromachiningapplications wherein a physical property is measurable. The measurementis not restricted to electrical measurements, but may be temperaturemonitoring (for instance, with an infrared sensor), stress, vibration,or other property.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

1. A method for high-speed, precise micromachining an array of devices,each of the devices having at least one measurable property, the methodcomprising the steps of: selectively laser micromachining a device inthe array along a linear path to vary a value of a measurable property;suspending the step of selectively laser micromachining; while the stepof selectively laser micromachining is suspended, selectively lasermicromachining at least one other device in the array along a pathcollinear with the linear path to vary a value of a measurable property;and resuming the suspended step of selectively micromachining to vary ameasurable property of the device until its value is within a desiredrange.
 2. The method of claim 1 wherein the devices are resistors. 3.The method of claim 2 wherein the resistors are film resistors.
 4. Themethod of claim 1 wherein the steps of selectively laser micromachiningare performed with at least one laser beam and a spot size less thanabout 30 microns which cuts the devices.
 5. The method of claim 1further comprising measuring one measurable property of at least one ofthe devices to obtain a measured value.
 6. The method of claim 5 furthercomprising comparing the measured value with a predetermined thresholdto obtain a comparison and laser micromachining at least one of theother devices based on the comparison.
 7. The method of claim 5 furthercomprising selectively laser micromachining at least one of the otherdevices based on the measured value.
 8. The method of claim 5 furthercomprising determining not to measure a measurable property of at leastone of the other devices based on the measured value.
 9. The method ofclaim 1 wherein the method is for high-speed, precise laser trimming anarray of resistors and wherein one measurable property is resistance.10. The method of claim 1 wherein each of the steps of selectively lasermicromachining includes the steps of generating and relativelypositioning a laser beam to travel in a first direction within a fieldof the array and selectively irradiating at least a portion of at leastone device within the field with at least one laser pulse to removematerial.
 11. The method of claim 1 wherein the array includes at leastone of one or more rows and one or more columns.
 12. The method of claim1 wherein at least one of the steps of selectively micromachining isperformed with a plurality of focused laser pulses to irradiate aplurality of devices substantially simultaneously.
 13. The method ofclaim 1 wherein at least one of the steps of selectively lasermicromachining is performed with a plurality of focused laser pulses andwherein the method further comprises distributing the focused laserpulses.
 14. The method of claim 13 wherein the step of distributingincludes the steps of producing a distribution pattern with a pluralityof laser beams and focusing the laser beams.
 15. The method of claim 9wherein the laser trimming produces a series of interdigited cuts in anarea of resistive material between conductors of the resistors.
 16. Themethod of claim 1 wherein at least one of the steps of selectively lasermicromachining includes the steps of positioning a laser beam at alocation of each of the devices to be micromachined and selectivelyirradiating at least a portion of each of the devices to bemicromachined with at least one laser pulse.
 17. The method of claim 1further comprising generating and relatively positioning a laser beam totravel in a first direction within a field of the array and selectivelyirradiating at least a portion of at least one device within the fieldwith at least one laser pulse and in a second direction substantiallyopposite the first direction within the field and selectivelyirradiating at least a second portion of at least one device within thefield with at least one laser pulse.
 18. The method of claim 1 whereinat least one of the steps of selectively laser micromachining includesthe steps of generating and relatively positioning a laser beam totravel in a first scanning pattern across the devices, superimposing asecond scanning pattern with the first scanning pattern and irradiatingat least one device with at least one laser pulse.
 19. The method ofclaim 18 wherein the second scanning pattern is a retrograde scan andwherein scan speed of the at least one laser pulse irradiating the atleast one device is lower than a corresponding scan speed of the firstscanning pattern wherein laser energy is concentrated at the at leastone device for a period of time longer than a period of time associatedwith only the first scanning pattern whereby throughput is improved. 20.The method of claim 1 wherein the steps of selectively lasermicromachining are performed with a plurality of laser pulses andwherein at least one of the pulses has an energy in the range of 0.1microjoules to 25 millijoules.
 21. The method of claim 7 wherein themeasured value is a measured temperature value.
 22. The method of claim1 wherein the devices are substantially identical.
 23. The method ofclaim 1 further comprising laser micromachining a sequence of cuts on aresistor based on a pre-determined parameter, a measured property or aknown property of the resistor, wherein the length of at least one cutis modified to prevent a processing condition in a subsequent cut. 24.The method of claim 1, wherein measurements of resistor width andalignment data are used for controlling a cut length.
 25. The method ofclaim 20 further comprising automatically controlling beam energy with avariable attenuator or modulator.